Liquid crystal displays (LCDs) are used in a wide variety of computers and consumer devices such as TVs. A back-lighted LCD is an array of pixels in which each pixel acts as a shutter that either passes or blocks light from a light source that is located behind the pixel. Color displays are implemented by equipping the pixels with color filters such that each pixel transmits or blocks light of a particular color. The intensity of the light from each pixel is set by the time the pixel is in the transmissive state.
The display is typically illuminated by a white light source that provides a uniform intensity of light across the back surface of the display. Illumination sources based on fluorescent lights are particularly attractive because of their high light output per watt-hour of power consumed. However, such sources require high driving voltages which makes them less attractive for battery operated devices.
As a result, there has been considerable interest in utilizing light sources based on LEDs in such applications. LEDs have similar electrical efficiency and long lifetimes. In addition, the driving voltages needed are compatible with the battery power available on most portable devices. An LED light source for generating an arbitrary color of light is typically constructed from three LEDs. The relative intensities of the LEDs are adjusted by adjusting the drive current through the LED and/or the duty factor of the LED. In the latter arrangement, the LEDs are turned on and off within a cycle time that is too short to be perceived by a human observer. The intensity of the light seen by the viewer is the average intensity, and hence, the relative intensities of the various colors are determined by the percentage of the time the various LEDs are turned on.
Back lighted illumination systems for LCD arrays typically utilize some form of light box or light pipe behind the LCD array. The light pipe is a rectilinear transparent solid constructed from a transparent plastic having one surface that has dimensions that are larger than the LCD array. The goal of the illumination system is to have this surface act as an extended light source having a uniform light intensity over the surface. Light is injected into the light pipe at the periphery of the light pipe. The light is trapped in the light pipe by internal reflections until the light is scattered in a manner that allows it to escape through the top surface, which is the surface that is adjacent to the LCD array. The bottom surface of the light box or the material of the light pipe itself has scattering centers that redirect the light hitting each center so that a portion of the light exits through the top surface.
In many applications, the size of the light source is an important factor. For small handheld devices, size is particularly important both in terms of the thickness of the light source and the amount of space required around the edges of the extended light source that is needed for the light source that illuminates the light box.
The thickness of the light source, and hence, the device is limited by the thickness of the light box. The thickness of the display is particularly important in displays used for laptop computers and handheld devices such as PDAs and cellular telephones, as the display thickness limits the overall thickness of the device. Some of these portable devices require light boxes that are less than 1 mm in thickness.
Light sources that are constructed from a light box that is illuminated along one or more edges by discrete packaged LEDs are limited both in terms of the thickness of the light source and the amount of edge space that is needed in addition to the surface that is being illuminated. As the thickness of the display decreases, the efficient injection of light into the light pipe becomes more problematic. Light must enter the edge of the light pipe at a predetermined point within a predetermined cone of angles. Typically, the light source consists of a number of packaged LEDs that are mounted on a small substrate such that the light emission direction of the LEDs is parallel to the surface of the light pipe. This substrate is attached to a circuit board that is under the light pipe such that light is emitted into the edge of the light pipe. If the relative positions of the light source and light pipe are not correct, part of the light can be lost either because the light misses the edge of the light pipe or because the angle at which some of the light enters the light pipe is greater than the critical angle, and hence, that light leaves the light pipe at the first reflection. In either case, the efficiency and/or the uniformity of the illumination system is reduced. In addition, when the thickness of the light pipe becomes less than the diameter of the LED package, providing good coupling of the light into the light pipe becomes even more difficult.
In addition, there is a minimum distance between the LEDs that is set by the diameter of the LED package. In a color light source, the LEDs are normally arranged as repeating red, green, and blue LEDs along the axis of the light source. The light entering the light source has hot spots immediately adjacent to the packaged LEDs both in terms of intensity and color. Hence, the region adjacent to the edge is reserved as a mixing region, and hence, the surface above this region is not useable as part of the extended light source.